1. Field of the Invention
The present invention relates to an electronic flash device which includes a first capacitor capable of storing the electrical energy required for emission means to effect an emission, and a second capacitor additionally connected to the first capacitor.
2. Description of Related Art
An electronic flash device is arranged to apply a charged voltage of several hundred volts stored in a main capacitor across a xenon tube (hereinafter referred to as the Xe tube) which constitutes emission means, and at the same time apply a trigger voltage of several thousand volts to a trigger electrode of the Xe tube, to excite the Xe tube to cause it to effect an emission. However, if the voltage of the main capacitor is low, it becomes impossible to cause the Xe tube to effect an emission by the application of a trigger voltage below a predetermined voltage, depending on the characteristics of the Xe tube. This predetermined voltage is hereinafter referred to as the emission starting voltage.
The emission starting voltage is determined by conditions such as the tube diameter and the arc characteristics of the Xe tube and the pressure of xenon gas, and is approximately 250 volts in the case of general Xe tubes employed in small-sized cameras. If the emission starting voltage is high, the electronic flash device cannot effect an emission until the voltage of the main capacitor becomes fully high, so that the charging of the main capacitor takes time and the response speed of shooting becomes inferior.
To solve the above problem, it has heretofore been proposed to use a circuit provided with a capacitor for applying a voltage for lowering the negative terminal voltage of a Xe tube to a ground level or below at the time of start of an emission. Such a known circuit which employs a voltage doubling capacitor and an insulated gate bipolar transistor (hereinafter referred to as IGBT) has been disclosed in Japanese Laid-Open Patent Application No. Sho 64-17033.
FIG. 6 is a circuit diagram showing the essential portion of a circuit illustrated in Japanese Laid-Open Patent Application No. Sho 64-17033. The shown circuit includes a main capacitor 200 for storing the energy required for emission and is charged by a power source (not shown), a coil 201, a diode 202, a Xe tube 203, resistors 204 and 205, a voltage doubling capacitor 206, a diode 207, a resistor 208 and an IGBT 209. If a voltage of high level is applied to a gate electrode GATE from a control circuit (not shown), the collector and the emitter of the IGBT 209 are electrically connected to each other, whereas if a voltage of low level is applied to the gate electrode GATE, the collector and the emitter of the IGBT 209 are electrically disconnected from each other.
In the above-described circuit, if the main capacitor 200 is charged by the power source (not shown), the voltage doubling capacitor 206 is charged in the shown polarity through the resistors 204 and 208. During an emission, a trigger circuit (not shown) applies a voltage of several thousand volts to a trigger electrode TRIG of the Xe tube 203 and at the same time applies a voltage of high level to the gate electrode GATE of the IGBT 209, thereby electrically connecting the emitter and the collector of the IGBT 209. At this time, the positive terminal (anode) of the voltage doubling capacitor 206 is lowered to a ground potential through the resistor 205 and the IGBT 209, so that a negative voltage is generated on the negative terminal (cathode) side of the voltage doubling capacitor 206 and the negative voltage is applied to the negative terminal side of the Xe tube 203. Thus, a voltage which is twice the charged voltage of the main capacitor 200 is applied across the Xe tube 203.
Another circuit which applies a higher voltage to the Xe tube 203 at the time of start of an emission by using a plurality of doubled voltage applying circuits of the above-described type is disclosed in Japanese Laid-Open Patent Application No. Hei 4-306594.
The conventional example disclosed in Japanese Laid-Open Patent Application No. Sho 64-17033 is arranged in such a manner that the voltage doubling capacitor 206 is charged through the resistor 204 and the resistor 208. If the Xe tube 203 needs to be made to effect an emission repeatedly at a high speed, the voltage doubling capacitor 206 needs to be charged at a high speed for each emission. However, if the resistance value of the resistor 208 is made small to reduce the charging time of the voltage doubling capacitor 206, an emission current will flow through the resistor 208 even when the IGBT 209 is turned off at the time of stop of an emission, so that the emission will not be stopped. It is, therefore, impossible to reduce the resistance value of the resistor 208 to approximately 100 K.OMEGA. or below. For this reason, in this conventional example, it is difficult to charge the voltage doubling capacitor 206 at a high speed and hence, it is impossible to cope with a high-speed repeated emission.
The conventional example disclosed in Japanese Laid-Open Patent Application No. Hei 4-306594, which is provided with a plurality of doubled voltage applying circuits, effectively works if the voltage of the main capacitor is low. However, if the voltage of the main capacitor is high, there is the problem that a voltage higher than necessary is applied across the Xe tube to adversely affect the durability, the wiring and the like of the Xe tube.
The electronic flash device shown in FIG. 7 is another known example. This device includes constituent elements such as a main capacitor 300 to be charged by a high voltage power source (not shown), a trigger capacitor 302 to be charged by the power source through a resistor 301, a trigger transformer 307 for applying a high voltage across a Xe tube 310 at the time of an emission, a thyristor 304 for trigger control, an IGBT 312 for emission control, and a voltage doubling capacitor 313 for applying a voltage which is twice the charged voltage of the main capacitor 300, across the Xe tube 310 during an emission.
In operation, the main capacitor 300 is charged through the high voltage power source (such as a DC/DC converter which is not shown), and the trigger capacitor 302 is also charged in the shown polarity up to the same voltage as the main capacitor 300 through the resistor 301. At the same time, the voltage doubling capacitor 313 is also charged up to the same voltage as the main capacitor 300 through the resistor 301, the diode 303 and the resistor 315.
In the case of emission, when a signal of high level is applied to the gate of the thyristor 304, the thyristor 304 is turned on, and the electric charge of the trigger capacitor 302 flows through the thyristor 304 and the trigger transformer 307 so that a high voltage is generated on the secondary winding side of the trigger transformer 307 to excite the Xe tube 310. At the same time, when a signal of high level is applied to the gate of the IGBT 312 to turn on the IGBT 312, the Xe tube 310 starts an emission.
During the emission, since the positive terminal of the voltage doubling capacitor 313 is grounded through the thyristor 304, the positive terminal potential of the voltage doubling capacitor 313 becomes zero and the negative terminal potential of the voltage doubling capacitor 313 becomes a negative potential. Accordingly, letting Vmc be the voltage of the main capacitor 300, the cathode potential of the Xe tube 310 is -Vmc and the anode potential of the Xe tube 310 is Vmc, i.e., a voltage which is twice the charged voltage of the main capacitor 300 is applied across the Xe tube 310, so that even if the voltage of the main capacitor 300 is low, the Xe tube 310 can readily effect an emission. When the emission is to be stopped, the gate of the IGBT 312 is set to a low level and the IGBT 312 is turned off to stop the emission of the Xe tube 310.
In the above-described conventional example, at the time of start of an emission, a voltage which is twice the voltage of the main capacitor 300 is applied across the Xe tube 310. However, the voltage of a trigger circuit which actually excites the Xe tube 310 is the same as the voltage of the main capacitor 300, and if the voltage of the main capacitor 300 is low, the voltage generated on the secondary winding side of the trigger transformer 307 also becomes low. This leads to what is called "lack of emission", i.e., the problem that the Xe tube 310 cannot effect an emission.